Silicon carbide whiskers have been proposed for use as reinforcements for light metals (e.g., aluminum and magnesium), ceramics and polymers. Materials so reinforced are referred to as metal matrix composites, ceramic matrix composites and polymer matrix composites, respectively.
Silicon carbide whiskers are typically made using either a vapor-solid reaction, often referred to as a V-S process, or a vapor-liquid-solid reaction, often referred to as a V-L-S process. In the V-S process, vapor containing silicon and carbon species react directly to form solid silicon carbide whiskers on a carbon (e.g., graphite) substrate. The V-L-S process is similar to the V-S process but also includes molten droplets of ferrous alloys on the carbon substrate which serve to initiate the silicon carbide whisker formation at the interface between the droplet and the carbon substrate. In both processes, the silicon carbide continues to grow from the tip of the previously formed silicon carbide.
Typically, V-L-S process-formed silicon carbide whiskers have diameters of about 1-3 microns, have lengths up to about 3 millimeters, have a smooth surface, form at a faster rate than V-S process-formed silicon carbide, and are characterized by a small bead of metal at the tip of the whisker. Silicon carbide formed by the V-S process tends to have diameters less than 1 micron, lengths of about 20 microns and a rougher surface than the V-L-S process-formed silicon carbide. Both the V-S and V-L-S processes are typically carried out at temperatures between 1400.degree. C. and 1600.degree. C. and are believed by many to result from the following reaction: EQU 3SiO(g)+CO(g).fwdarw.SiC.sub.(w) +2SiO.sub.2 (g)
It is an object of the present invention to provide a high temperature (i.e., 1600.degree. C.-1800.degree. C.) V-S type process which rapidly grows long (i.e., aspect ratios as high as 200:1), smooth silicon carbide whiskers having average diameters greater than about 1 micron which process is carried out in a nitrogenous atmosphere in the presence of sulfur vapor and a group II metal vapor selected from the group consisting of calcium, magnesium and barium. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows and which is described hereafter in conjunction with the several Figures in which:
FIG. 1 illustrates a sectioned side view of one embodiment of a reactor for carrying out the process of the present invention;
FIG. 2 is a sectioned side view of another embodiment of a reactor for carrying out the process of the present invention;
FIG. 3 is a sectioned side view of another embodiment of a reactor for carrying out the process of the present invention;
FIG. 4 is a sectioned side view of still another embodiment of a reactor for carrying out the process of the present invention; and
FIGS. 5-12 are histograms for silicon carbide whiskers grown under various conditions within the scope of the present invention.